A number of different products include audio circuitry, such as an audio amplifier, together with one or more loudspeakers and/or connections for driving one or more loudspeakers of an integrated apparatus such as a mobile phone, i.e. handset, and/or peripheral apparatus such as a headset, e.g. earbuds, headphones, hearing aids and Bluetooth™ devices. In some instances the loudspeaker(s) chosen will be robust enough and sized sufficiently to handle the maximum power level at which the amplifier could drive signals continuously into it, even under the worst case environmental conditions, for instance maximum supply voltage, maximum ambient temperature etc. However having robust enough loudspeakers is not always economical, and for portable devices such as mobile phones or tablets and headsets and the like the desire is typically to make the speaker as small and light as possible. This can potentially lead to the audio drive circuitry overloading the loudspeaker. One particular problem is mechanical damage due to excessive and/or prolonged displacement or excursion of the speaker mechanism.
It is known to provide circuitry to estimate the displacement of a speaker over time from the voltage applied to the speaker using a plant model, i.e. a model of how the speaker reacts, and to reduce the applied signal if over-excursion is predicted. FIG. 1 illustrates a speaker driver system for driving loudspeaker 10 via output amplifier circuitry 11. The system receives, at an input node IN, an input signal V representing the output voltage Vout to be driven into the loudspeaker 10. The input signal V is also applied to an excursion prediction block 12 which uses a plant model to provide an estimate x of the speaker coil excursion. Gain calculator block 13 then compares this predicted excursion x with some predefined excursion or displacement limit xlim. If the estimated excursion x exceeds the limit xlim then a reduced gain value is calculated, for example equal to xlim/xi, and a corresponding reduced gain is applied to the input signal, e.g. by a gain adjustment element 14 that is upstream of the loudspeaker in the signal path, so as to attenuate the input signal and hence reduce the speaker excursion to less than the limit xlim.
The loudspeaker 10 may have substantial time constants associated with its mechanical inertia and the inertia of the air volume to be driven. This may be expressed in terms of the settling time of its impulse response which may extend over several milliseconds. There will thus be a delay between when the input signal V is received by the speaker 10 and when the full extent of its effect on the voice coil excursion is experienced and thus recognised by the excursion prediction block 12. Thus the input signal V is passed via a delay block 15 to apply a delay d in the signal path between the signal input node IN and the gain adjustment element 14 so that the reduced gain may be applied to a delayed version Vd of the input signal stream early enough, i.e. to the portion of the signal stream Vd that actually would have produced the over-excursion if it had been allowed to be applied to the loudspeaker 10.
This delay d may be acceptable in some applications, but in some applications such as telephony, e.g. wireless communication, especially in speakerphone type operation, such a delay d might cause problems and therefore be disadvantageous.